African elephants may be the shortest-sleeping mammals

African elephants in the wild sleep an average of two hours a day and regularly go nearly two days without sleep, according to a study published March 1, 2017 in the open-access journal PLOS ONE by Paul Manger from University of the Witwatersrand, South Africa, and colleagues.

African elephants are the largest land animal, and evidence suggests that larger mammals tend to sleep less. However, many studies on elephant sleep have been done in a captive setting or were unable to accurately distinguish rest from sleep. To study in more detail how elephants sleep in the wild, Manger and colleagues monitored two free-roaming African elephant matriarchs in Chobe National Park, Botswana, for 35 days. The researchers outfitted the elephants with an actiwatch implanted in the trunk to track sleep accurately, and a collar with a gyroscope to track sleeping position.

The researchers found that the elephants slept an average of two hours a day, which is the shortest known sleep time of any land mammal. On several days during the study period, the elephants went without sleep for up to 46 hours and traveled long distances of around 30 kilometers during these periods, possibly due to disturbances such as lions or poachers. In addition, they slept lying down only every few nights. This could limit their potential for daily REM (rapid eye movement) sleep, raising questions about when elephants experience this sleep state. While only two elephants were tracked, this research provides new insights into the sleep behavior of the species in the wild.

"Studies of sleep in captive elephants have shown that they sleep for four to six hours per day; however, the current study shows that in their natural habitat, wild, free-ranging elephants sleep only for two hours per day, the least amount of sleep of any mammal studied to date, but this appears to be related to their large body size," says Manger. "In addition, it appears that elephants only go into REM, or dreaming, sleep every three to four days, which makes elephant sleep unique."

Funding: This research was funded by the National Research Foundation of South Africa (http://www.nrf.ac.za) (reference: CPR20110602000019125, grant number: 81822, awarded to PRM). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.

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